1. Field of the Invention
The present invention relates to a characteristic monitoring method of a pumping light source for optical amplification for when a pumping light is supplied to a rare earth doped fiber to amplify a signal light, and an optical amplifier. In particular, the present invention relates to a technique for monitoring an optical power characteristic of a pumping light source using a semiconductor laser without affecting the amplification of signal light, to reflect the monitoring result in a pumping light control of an optical amplifier.
2. Description of the Related Art
As an optical amplifier for amplifying a wavelength division multiplexed (WDM) light containing a plurality of signal lights having different wavelengths, there has been known an optical amplifier utilizing an optical fiber doped with a rare earth element, for example. This optical amplifier using the rare earth element doped fiber is for supplying a pumping light output from a pumping light source using, for example, a semiconductor laser or the like, to the rare earth element doped fiber, to amplify the WDM light up to a desired level by stimulated emission which occurs when the WDM light is propagated through the rare earth element doped fiber in a pumped state (refer to Japanese Unexamined Patent Publication Nos. 5-55673 and 8-204267).
It has been known that the pumping light source used for the above optical amplifier, is given with a drive signal generated by a drive circuit, to output a pumping light of required power, however, an optical power characteristic of the pumping light source relative to the drive signal is changed due to a temperature variation or the deterioration with time. Particularly, in recent years, a so-called cooler-less semiconductor laser in which a temperature adjustment function is omitted, has been utilized as a pumping light source, bringing the reduction of power consumption and the cost reduction into view. In such a case, an influence on the signal light amplification due to a change in the optical power characteristic of the pumping light source becomes large. Accordingly, in order to obtain the WDM light amplified up to the desired level by the optical amplifier, it is needed to control the drive signal according to a change in characteristic of the pumping light source.
For the drive control of the pumping light source in the conventional optical amplifier, there has been proposed a technique in which, on the basis of data for outputting a required pumping light according to the ambient temperature, with a pumping light for when the pumping light source is driven in accordance with the data being a reference value, a pumping light at the operation time and the reference value are compared with each other, to thereby perform the temperature compensation and the compensation for deterioration with time of the pumping light source (refer to Japanese Unexamined Patent Publication No. 2002-217836).
Further, to the optical amplifier using the rare earth element doped fiber as described above, an automatic level control (ALC) controlling a level of output light to be fixed or an automatic gain control (AGC) controlling a gain to be fixed is typically applied (refer to Japanese Unexamined Patent Publication No. 10-209970).
FIG. 8 is a block diagram showing one example of a conventional optical amplifier applied with the AGC. In this optical amplifier, for example, a pumping light output from a pumping light source (LD) 102 is supplied to an erbium doped fiber (EDF) 101 via a multiplexer 103, and a part of a WDM light that is to be input to the EDF 101 is demultiplexed by a demultiplexer 104, to be photo-electrically converted by a light receiving element (PD) 105, so that the input light power is monitored. Also, a part of the WDM light output from the EDF 101 is demultiplexed by a demultiplexer 106, to be photo-electrically converted by a light receiving element (PD) 107, so that the output light power is monitored. Then, the respective monitoring results are sent to an AGC circuit 108 in which an amplification degree in the EDF 101 is calculated, and a drive condition of the pumping light source 102 is controlled according to the calculation result, so that a fixed gain can be obtained. By performing the AGC by such a control circuit, it becomes possible to suppress an occurrence of gain deviation between signal lights having respective wavelengths contained in the WDM light.
In a WDM optical transmission system to which the conventional optical amplifier as described above is applied, there are, for example, the case where the number of wavelengths of signal lights contained in a WDM light is increased with an increase in transmission data, addition of transmission system or the like, or the case where the number of signal light wavelengths is decreased for maintenance or the like. It is required that the operational wavelength is not affected even when the number of signal light wavelengths is increased or decreased. Especially, for example, in a system with the adding/dropping of signal light as shown in FIG. 9, in the case where a fault, such as breakage of transmission path fiber or the like, occurs, there is a possibility that the number of signal light wavelengths is significantly changed, such as, from a maximum n+1 waves to 1 wave. If the number of signal light wavelengths is abruptly changed as described above, in an optical amplifier 100B located downstream the fault occurring point, since the optical amplification is usually performed in a gain saturation region, there occurs a large level variation in the remaining signal light, in the AGC at a low speed.
Here, the description will be made on a transient response phenomenon of an optical amplifier, which occurs due to a change in the number of signal light wavelengths. Note, a transient response means a transient progress exhibited after a response is generated due to an input given to a control system until the response reaches in a new steady state. In an example in which a pumping light control cannot promptly cope with the change in the wavelength number of WDM light input to the optical amplifier (the level change in input signal light), the transient response described above appears as an optical surge to cause a transmission error.
For example, when a fault, such as transmission path fiber breakage or the like, occurs in the system shown in FIG. 9, and the transmission of signal lights having wavelengths λ1 to λn is interrupted, it is required that the transmission error does not occur in the signal light having wavelength λn+1 to be added subsequently, even if the signal lights having wavelengths λ1 to λn are not input to the optical amplifier. In order to satisfy this requirement, in the optical amplifier, it is necessary to immediately reduce the power of pumping light from the power corresponding to n+1 waves to the power corresponding to 1 wave, to amplify the signal light having wavelength λn+1 with a pumping light corresponding to 1 wave.
However, since the following capability of the conventional AGC at the time when the number of signal light wavelengths is changed, as shown in FIG. 10 for example, although only the signal light of 1 wave is input, a period of time becomes longer during which the pumping light equivalent to n+1 waves is given to the rare earth element doped fiber, resulting in that the gain is abruptly varied, and the remaining light at a high level (optical surge) is generated instantaneously from the output of the optical amplifier. This optical surge is transmitted to cause the transmission error, and in a system in which the optical amplifiers are connected in multi-stages, the optical surges are accumulated and are amplified. Therefore, there is a possibility that the receiver is damaged. In order to solve such a problem, it is required to apply a high speed AGC, which does not substantially change an inside state (population inversion) of the rare earth element doped fiber.
Further, for the following capability of the AGC at the time when the number of signal light wavelengths is changed, it becomes important that a proportional factor of the control circuit is optimized according to the optical power characteristic of the pumping light source. Namely, for example, when the cooler-less semiconductor laser is utilized as the pumping light source as described above, the optical power characteristic (I-L characteristic) of the semiconductor laser relative to the drive current is significantly changed due to a variation of ambient temperature. To be specific, as shown in an I-L characteristic exemplified in FIG. 11, a slope (slope efficiency) of the I-L characteristic for when the drive current exceeding an oscillation threshold is given to the semiconductor laser, is changed by 1.5 times due to the temperature variation. The fact that the slope of the I-L characteristic of the semiconductor laser is changed by 1.5 times means that the proportional factor of the AGC circuit is changed by 1.5 times, which affects the following capability of the AGC at the time when the number of signal light wavelengths is abruptly changed as described above.
Specifically, FIG. 12 exemplarily shows how the differences show in a level variation of the remaining signal light having 1 wave in the case where the proportional factor of the AGC circuit is changed, when there occurs the change in the number of signal light wavelengths as shown in FIG. 10. A transverse axis of FIG. 12 indicates a period of time during which the total input power to the optical amplifier is reduced from 90% to 10%, namely, a speed of the change in the number of signal light wavelengths. A vertical axis of FIG. 12 indicates a variation amount of the peak power of the remaining signal light having 1 wave. Here, with the proportional factor of a typical AGC circuit being a reference (one time), the proportional factor is reduced to ⅔ times. In other words, the comparison is performed on the level variation for when the slope of the I-L characteristic of the pumping light source is changed by ⅔ times. As shown in FIG. 12, it is understood that, if the proportional factor of the AGC circuit is reduced, the level variation of the remaining signal light becomes larger. The reason why such a difference occurs is that, if the proportional factor is reduced, the speed for reducing the drive current for the pumping light source is dropped when the number of signal light wavelengths is reduced.
Accordingly, in order to avoid an influence on the remaining signal light even when the number of signal light wavelengths is abruptly changed, it becomes important that the optical power characteristic of the pumping light source can be monitored with high accuracy at the operation time of the optical amplifier, and the proportional factor of the AGC circuit can be corrected according to the monitoring result. However, in the conventional technique disclosed in each prior art references described above, it has been difficult to solve the above problems.
For the technique for monitoring the optical power characteristic of the pumping light source used in the optical amplifier, the present applicant has proposed a technique for changing the drive current for the pumping light source and measuring regularly the pumping light power supplied to an amplification medium, to detect a characteristic change in the pumping light source (refer to Japanese Unpublished Patent Application No. 2003-57951). However, even in this prior application, there still remains a problem as to how the drive current for the pumping light source is changed at the operation time of the optical amplifier. That is, in the case where the drive current for the pumping light source is changed at the operation time of the optical amplifier, there is a possibility that the amplification of the signal light in the operation is affected due to such a change in the drive current. Therefore, it is required to realize a specific monitoring method which avoids such a possibility.